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Subject[PATCH 29/36] blkcg: drop unnecessary RCU locking
Date
Now that blkg additions / removals are always done under both q and
blkcg locks, the only places RCU locking is necessary are
blkg_lookup[_create]() for lookup w/o blkcg lock. This patch drops
unncessary RCU locking replacing it with plain blkcg locking as
necessary.

* blkiocg_pre_destroy() already perform proper locking and don't need
RCU. Dropped.

* blkio_read_blkg_stats() now uses blkcg->lock instead of RCU read
lock. This isn't a hot path.

* Now unnecessary synchronize_rcu() from queue exit paths removed.
This makes q->nr_blkgs unnecessary. Dropped.

* RCU annotation on blkg->q removed.

-v2: Vivek pointed out that blkg_lookup_create() still needs to be
called under rcu_read_lock(). Updated.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Vivek Goyal <vgoyal@redhat.com>
---
block/blk-cgroup.c | 20 +++++++-------------
block/blk-cgroup.h | 4 ++--
block/blk-throttle.c | 33 +--------------------------------
block/cfq-iosched.c | 24 ------------------------
include/linux/blkdev.h | 1 -
5 files changed, 10 insertions(+), 72 deletions(-)
diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
index aee71ef..5cabdb8 100644
--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c
@@ -500,7 +500,7 @@ static struct blkio_group *blkg_alloc(struct blkio_cgroup *blkcg,
return NULL;

spin_lock_init(&blkg->stats_lock);
- rcu_assign_pointer(blkg->q, q);
+ blkg->q = q;
INIT_LIST_HEAD(&blkg->q_node);
blkg->blkcg = blkcg;
blkg->refcnt = 1;
@@ -611,7 +611,6 @@ struct blkio_group *blkg_lookup_create(struct blkio_cgroup *blkcg,

hlist_add_head_rcu(&blkg->blkcg_node, &blkcg->blkg_list);
list_add(&blkg->q_node, &q->blkg_list);
- q->nr_blkgs++;

spin_unlock(&blkcg->lock);
out:
@@ -648,9 +647,6 @@ static void blkg_destroy(struct blkio_group *blkg)
list_del_init(&blkg->q_node);
hlist_del_init_rcu(&blkg->blkcg_node);

- WARN_ON_ONCE(q->nr_blkgs <= 0);
- q->nr_blkgs--;
-
/*
* Put the reference taken at the time of creation so that when all
* queues are gone, group can be destroyed.
@@ -1224,8 +1220,9 @@ static int blkio_read_blkg_stats(struct blkio_cgroup *blkcg,
struct hlist_node *n;
uint64_t cgroup_total = 0;

- rcu_read_lock();
- hlist_for_each_entry_rcu(blkg, n, &blkcg->blkg_list, blkcg_node) {
+ spin_lock_irq(&blkcg->lock);
+
+ hlist_for_each_entry(blkg, n, &blkcg->blkg_list, blkcg_node) {
const char *dname = dev_name(blkg->q->backing_dev_info.dev);
int plid = BLKIOFILE_POLICY(cft->private);

@@ -1241,7 +1238,8 @@ static int blkio_read_blkg_stats(struct blkio_cgroup *blkcg,
}
if (show_total)
cb->fill(cb, "Total", cgroup_total);
- rcu_read_unlock();
+
+ spin_unlock_irq(&blkcg->lock);
return 0;
}

@@ -1573,28 +1571,24 @@ static int blkiocg_pre_destroy(struct cgroup_subsys *subsys,
{
struct blkio_cgroup *blkcg = cgroup_to_blkio_cgroup(cgroup);

- rcu_read_lock();
spin_lock_irq(&blkcg->lock);

while (!hlist_empty(&blkcg->blkg_list)) {
struct blkio_group *blkg = hlist_entry(blkcg->blkg_list.first,
struct blkio_group, blkcg_node);
- struct request_queue *q = rcu_dereference(blkg->q);
+ struct request_queue *q = blkg->q;

if (spin_trylock(q->queue_lock)) {
blkg_destroy(blkg);
spin_unlock(q->queue_lock);
} else {
spin_unlock_irq(&blkcg->lock);
- rcu_read_unlock();
cpu_relax();
- rcu_read_lock();
spin_lock(&blkcg->lock);
}
}

spin_unlock_irq(&blkcg->lock);
- rcu_read_unlock();
return 0;
}

diff --git a/block/blk-cgroup.h b/block/blk-cgroup.h
index bebc442..1a80619 100644
--- a/block/blk-cgroup.h
+++ b/block/blk-cgroup.h
@@ -176,8 +176,8 @@ struct blkg_policy_data {
};

struct blkio_group {
- /* Pointer to the associated request_queue, RCU protected */
- struct request_queue __rcu *q;
+ /* Pointer to the associated request_queue */
+ struct request_queue *q;
struct list_head q_node;
struct hlist_node blkcg_node;
struct blkio_cgroup *blkcg;
diff --git a/block/blk-throttle.c b/block/blk-throttle.c
index e35ee7a..bfa5168 100644
--- a/block/blk-throttle.c
+++ b/block/blk-throttle.c
@@ -1046,39 +1046,8 @@ int blk_throtl_init(struct request_queue *q)

void blk_throtl_exit(struct request_queue *q)
{
- struct throtl_data *td = q->td;
- bool wait;
-
- BUG_ON(!td);
-
+ BUG_ON(!q->td);
throtl_shutdown_wq(q);
-
- /* If there are other groups */
- spin_lock_irq(q->queue_lock);
- wait = q->nr_blkgs;
- spin_unlock_irq(q->queue_lock);
-
- /*
- * Wait for tg_to_blkg(tg)->q accessors to exit their grace periods.
- * Do this wait only if there are other undestroyed groups out
- * there (other than root group). This can happen if cgroup deletion
- * path claimed the responsibility of cleaning up a group before
- * queue cleanup code get to the group.
- *
- * Do not call synchronize_rcu() unconditionally as there are drivers
- * which create/delete request queue hundreds of times during scan/boot
- * and synchronize_rcu() can take significant time and slow down boot.
- */
- if (wait)
- synchronize_rcu();
-
- /*
- * Just being safe to make sure after previous flush if some body did
- * update limits through cgroup and another work got queued, cancel
- * it.
- */
- throtl_shutdown_wq(q);
-
kfree(q->td);
}

diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
index 354a6eb..42fa071 100644
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -3449,7 +3449,6 @@ static void cfq_exit_queue(struct elevator_queue *e)
{
struct cfq_data *cfqd = e->elevator_data;
struct request_queue *q = cfqd->queue;
- bool wait = false;

cfq_shutdown_timer_wq(cfqd);

@@ -3462,31 +3461,8 @@ static void cfq_exit_queue(struct elevator_queue *e)

spin_unlock_irq(q->queue_lock);

-#ifdef CONFIG_BLK_CGROUP
- /*
- * If there are groups which we could not unlink from blkcg list,
- * wait for a rcu period for them to be freed.
- */
- spin_lock_irq(q->queue_lock);
- wait = q->nr_blkgs;
- spin_unlock_irq(q->queue_lock);
-#endif
cfq_shutdown_timer_wq(cfqd);

- /*
- * Wait for cfqg->blkg->key accessors to exit their grace periods.
- * Do this wait only if there are other unlinked groups out
- * there. This can happen if cgroup deletion path claimed the
- * responsibility of cleaning up a group before queue cleanup code
- * get to the group.
- *
- * Do not call synchronize_rcu() unconditionally as there are drivers
- * which create/delete request queue hundreds of times during scan/boot
- * and synchronize_rcu() can take significant time and slow down boot.
- */
- if (wait)
- synchronize_rcu();
-
#ifndef CONFIG_CFQ_GROUP_IOSCHED
kfree(cfqd->root_group);
#endif
diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index b4d1d4b..33f1b29 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -365,7 +365,6 @@ struct request_queue {
#ifdef CONFIG_BLK_CGROUP
/* XXX: array size hardcoded to avoid include dependency (temporary) */
struct list_head blkg_list;
- int nr_blkgs;
#endif

struct queue_limits limits;
--
1.7.7.3


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